Migration inhibitory factor (MIF), Type II interferon, and other lymphokines have been released in vivo into the circulation of mice with delayed hypersensitivity. Such mice are first sensitized, and then after an appropriate interval, challenged intravenously with the specific antigen. Bleeding of the mice 2-3 hours later and assay of the sera reveal the presence of active lymphokines. At first, only living Mycobacterium tuberculosis strain BCG injected intravenously was effective in inducing the proper degree of delayed hypersensitivity. More recently, nonviable BCG, other strains and species, and purified fractions of BCG have been found capable of inducing the proper sensitization, so that introduction of antigen results in subsequent release of lymphokines. In the regulatory action of circulating antibody on delayed hypersensitivity, the possible presence of a regulatory B-cell is being investigated, with a system involving the use of purified proteins as antigens in guinea pigs.